**Core Concept**
The patient described is exhibiting negative symptoms of schizophrenia, characterized by a lack of motivation, social withdrawal, and a flat affect. Negative symptoms are distinct from positive symptoms, such as delusions and hallucinations, and are often more challenging to treat.

**Why the Correct Answer is Right**
The patient's presentation suggests a need for an atypical antipsychotic with a strong efficacy profile for negative symptoms. Clozapine is a second-generation antipsychotic that has been shown to be effective in treating treatment-resistant schizophrenia, including patients with prominent negative symptoms. Clozapine's mechanism of action involves antagonism of dopamine D2 and serotonin 5-HT2 receptors, as well as agonism of serotonin 5-HT1A receptors.

**Why Each Wrong Option is Incorrect**
**Option A:** Haloperidol is a first-generation antipsychotic that is primarily used to treat positive symptoms of schizophrenia, such as delusions and hallucinations. It is not typically effective for negative symptoms and may even exacerbate them.
**Option B:** Olanzapine is an atypical antipsychotic that is effective for a wide range of symptoms, including positive and negative symptoms. However, it is not as effective as clozapine for treatment-resistant schizophrenia with prominent negative symptoms.
**Option C:** Risperidone is another atypical antipsychotic that is effective for both positive and negative symptoms. However, it may not be as effective as clozapine for patients with treatment-resistant schizophrenia.

**Clinical Pearl / High-Yield Fact**
Clozapine is often reserved for patients with treatment-resistant schizophrenia, but its use is not limited to this population. It is also effective in patients with prominent negative symptoms, such as the one described in the question.

**Correct Answer:** C. Clozapine